The ZX Spectrum 16k/48kexpansion connector is a double sided card edge connector with a 0.1 inch spacing. The two rows of conductors are numbered from right to left looking into the rear of the computer.
One pair of conductors are missing as there is an indexing slot cut out of the circuit board.

Contents

Notes on Connections

CPU Clock

The !CK signal, sometimes referred to as PHICPU is available on Lower Pin 8. This clock signal is generated by the ULA and is interrupted during contended memory access. This clock signal is inverted by a transistor switch to provide a clean clock edge for the Z80.

IO Request Signals

The !IORQ signal generated by the Z80 is connected to the !IOREQ input of the ULA via a series resistor allowing the !IOREQ pin to be pulled high by TR6 when the A0 address line is high. This has the effect of allowing the ULA to respond to an IO request only when A0 is low.

This combined !IORQ+A0 signal is connected to Lower Pin 13 and is referred to as !IORQULA or sometimes !IORQGE.

Some peripherals use this as an input to inhibit the ULA !IOREQ even when A0 is low thereby allowing the use of even numbered ports.

Key Slot

The key slot ensures correct alignment of a peripheral with the edge connector. This slot is the width of one conductor and lies between Pin 4 and Pin 6, i.e. Pin 5 does not exist.